THE SEASONAL OCCURRENCE, SOIL DISTRIBUTION AND FLIGHT CHARACTERISTICS OF CURCULIO SAYI
(COLEOPTERA: CURCULIONIDAE) IN MID-MISSOURI
__________________
A Thesis
Presented to
The Faculty of the Graduate School
University of Missouri – Columbia
_____________________
In Partial Fulfillment
Of the Requirements for the Degree
Master of Science
____________________
By
IAN W. KEESEY
Thesis Supervisor: Bruce A. Barrett
October 2007
The undersigned, appointed by the Dean of the Graduate School, have examined the
thesis entitled:
THE SEASONAL OCCURRENCE, SOIL DISTRIBUTION AND FLIGHT CHARACTERISTICS OF CURCULIO SAYI
(COLEOPTERA: CURCULIONIDAE) IN MID-MISSOURI
Presented by Ian W. Keesey
A candidate for the degree of Master of Science
And hereby certify that in their opinion it is worthy of acceptance.
______________________________________
______________________________________
______________________________________
______________________________________
ACKNOWLEDGEMENTS
The research completed over the course of this study would not have been
possible without the help of many individuals. I would first like to thank my major
advisor, Dr. Bruce Barrett, as his insights and suggestions while preparing this
manuscript were vital to its completion. Moreover, I would like to thank him for his
many years of support, advice, guidance and encouragement.
I would like to thank those at the Horticulture and Agroforestry Research Center
(HARC), especially Terry Woods and Randy Theissen, for their assistance in this project.
I would also like to thank Dr. Ken Hunt, who was always willing to give advice and grant
access to chestnuts, and without his expertise and associations with state nut growers this
project might not have been a success. Dr. W. Terrell Stamps played an essential role in
handling the gambit of questions associated with my research, both in the field and in the
laboratory, and I would like to express my thanks for his continued patience and
assistance.
Additionally, I have a special thank you for my committee members, Drs. Marc
Linit, Michael Gold and Qisheng Song. I consider myself very fortunate to have had
such an outstanding committee.
Finally, I would like to thank my family and friends for their support and
encouragement along the way. They have always been there for me, without them I
would not be who I am today.
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TABLE OF CONTENTS ACKNOWLEDGMENTS ................................................................................................... i LIST OF TABLES............................................................................................................. iv LIST OF FIGURES ........................................................................................................... iv CHAPTER I. Literature Review..................................................................................... 1
A. Host Plant.................................................................................................................. 1 1. Chestnut ................................................................................................................. 1 2. Chestnut blight ....................................................................................................... 2 3. Current chestnut tree research................................................................................ 4
B. Biology and Taxonomy of Chestnut Weevil Pests.................................................... 6 1. General descriptions of Curculio ........................................................................... 6 2. Life history of the lesser chestnut weevil (Curculio sayi) ..................................... 8 3. Life history of the greater chestnut weevil (Curculio caryatrypes)....................... 9 4. Life history of the European chestnut weevil (Curculio elephas) ....................... 10 5. Life history of the Italian chestnut weevil (Curculio propinquus) ...................... 11
C. Tree and Nut Damage by Curculio spp................................................................... 12 CHAPTER II. The Seasonal Occurrence of the Adult Lesser Chestnut Weevil, Curculio sayi, in Mid- Missouri............................................................ 14
A. Introduction............................................................................................................. 14 B. Materials and methods ............................................................................................ 15
1. Field site.............................................................................................................. 15 2. Cone traps ........................................................................................................... 15 3. Tree-mounted circle traps ................................................................................... 17 4. Pyramid traps ...................................................................................................... 17 5. Data analysis ....................................................................................................... 18
C. Results ..................................................................................................................... 18 1. Adult emergence data .......................................................................................... 18 2. Adult activity data................................................................................................ 19
D. Discussion ............................................................................................................... 20 CHAPTER III. Within-Soil Distribution and Development of the Lesser Chestnut Weevil, Curculio sayi ......................................................................... 32
A. Introduction............................................................................................................. 32 B. Materials and Methods ............................................................................................ 33
1. Field site............................................................................................................... 33 2. Soil cage containers ............................................................................................. 33 3. Insects .................................................................................................................. 33
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4. Sampling procedures and schedule...................................................................... 34 5. Data analysis ........................................................................................................ 35
C. Results ..................................................................................................................... 35 D. Discussion ............................................................................................................... 37
CHAPTER IV. Assessment of the Flight Characteristics of the Lesser Chestnut Weevil, Curculio sayi, from a Flight Mill ........................................... 47
A. Introduction............................................................................................................. 47 B. Materials and Methods ............................................................................................ 48
1. Insects .................................................................................................................. 48 2. Flight mill and procedures ................................................................................... 49
C. Results ..................................................................................................................... 50 D. Discussion ............................................................................................................... 51
CHAPTER V. Thesis Summary and Conclusions........................................................ 60 LITERATURE CITED ..................................................................................................... 62 VITA................................................................................................................................. 67
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LIST OF TABLES 1. Mean flight parameters of male and female Curculio sayi on a flight mill ................. 58
LIST OF FIGURES 1. Schematic pattern and dimensions of the ground emergence cone traps..................... 23 2. Commercial boll weevil trap top (disassembled) placed on top of the cone trap ........ 24 3. Emergence traps: ground-based cone trap fully assembled......................................... 25 4. Schematic depiction of the location of the five selected chestnut trees at the field site, as well as the locations of the emergence and activity traps........................................ 26 5. Activity traps: tree-mounted circle traps...................................................................... 27 6. Activity traps: pyramid trap fully assembled............................................................... 28 7. Numbers of adult Curculio sayi captured emerging from the ground in 2005............ 29 8. Numbers of adult Curculio sayi captured emerging from the ground and captured crawling up the tree trunks in 2006.............................................................................. 30 9. Numbers of adult Curculio sayi captured emerging from the ground and captured crawling up the tree trunks in 2007.............................................................................. 31 10. Schematic diagram of soil cage assembly showing the seven ring design ................ 39 11. A soil cage container assembly both above and below ground ................................. 40 12. Number of Curculio sayi larvae recovered from a single soil cage container (November 2005) ....................................................................................................... 41 13. Mean number of Curculio sayi larvae recovered from five soil cage containers (January 2006) ........................................................................................................... 42 14. Mean number of Curculio sayi larvae recovered from five soil cage containers (May 2006)................................................................................................................. 43 15. Number of Curculio sayi larvae and pupae recovered from one soil cage container (April 2007) ............................................................................................................... 44 16. Number of Curculio sayi adults emerging from four soil cage containers (May 2007)................................................................................................................. 45
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17. Mean number of Curculio sayi larvae, pupae and adults recovered from four soil cage containers (October 2007) ................................................................................. 46 18. Flight mill apparatus displaying the tethering methods used for both male and female Curculio sayi adults ................................................................................................... 55 19. Top view of entire flight mill apparatus with the associated light/heat sources and alternating black and white stripes............................................................................. 56 20. Schematic of the tether attachment method used to for adult Curculio sayi ............. 57 21. Flight speed over time of male and female Curculio sayi that had flown for approximately 2 hours................................................................................................ 59
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CHAPTER 1
Literature Review
A. Host Plant 1. Chestnut
The chestnut tree (Castanea spp.) is a deciduous plant that belongs to the family
Fagaceae (or beech family) and is native to warm temperate regions of the Northern
Hemisphere (Garcia-Carbonell et al. 2002, Higaki 2005). Due to the wide variation in
tree grafting and genetic recombination of chestnut trees as well as variability due to
environmental conditions, there is not an accurate time table that applies to all chestnut
tree development. Generally, chestnut trees take eight to ten years to reach the
reproductive stage where commercial nuts are produced (Davelos and Jarosz 2004,
Bounous and Marinoni 2005). However, cross breeding to produce a chestnut tree
capable of nut production in as little as six years is possible (Ken Hunt [University of
Missouri]). After reaching sexual maturity, chestnut flowers (or catkins) begin blooming
at the end of May in Mid-Missouri and continue through most of June, though some
cultivars do not finish flowering until early July (Ken Hunt [University of Missouri],
Soltész et al. 2003). Variation in time and duration of florescence occurs among the
many cultivars, and some chestnut trees produce a second flowering during the summer
that creates a second set of burs and fruit.
The fruit, or cupule, of the chestnut tree is encompassed by a spiny covering
known as the bur that surrounds the developing nuts. At maturity the bur will split open,
releasing the nuts that then fall to the ground. The nut or fruit from a chestnut tree are
distinct based on their geographical origin. For example, the American chestnut
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(Castanea dentata) tends to produce nuts that are smaller in size (~6g), and densely
covered in fine pubescence or hair over 1/3 to 2/3 of its surface area, whereas the Chinese
(Castanea mollissima) and European chestnut (Castanea sativa) produce much larger
fruit (up to 30g) with relatively few hairs (present perhaps only at the tip of the nut).
Chinquapin (Castanea pumila), though closely resembling chestnuts, will only have one
fruit or nut per bur, while chestnut burs contain 2-3 fruit. Chinquapin nuts, though
covered in hair like its chestnut cousin, also tend to be much smaller than the American
chestnut fruit, and the chinquapin nut has a distinct, pointed tip (Wells and Payne 1975,
Miller 2003).
The natural range of the American chestnut tree extended from the lower parts of
Canada to Georgia and from the Atlantic seaboard to Indiana. It was the dominant tree
along the Appalachian Mountains (Anagnostakis 2005). Prior to 1904, an estimated 25-
30% of timber acreage in the Eastern United States was American chestnut (Davelos and
Jarosz 2004, Anagnostakis 2005). This high density of American chestnut across the
Eastern United States could produce a large volume of nuts for wildlife and human
consumption each year. Native Americans, early settlers and more modern street vendors
used this abundant crop for human consumption. Many literature examples refer to the
classical phrase of “roasting chestnuts” (Johnson 1956, McEowen 2005, Hunt et al.
2006). However, the high density of the American chestnut over the eastern United
States would also prove to be a factor in the demise of the tree, as a disease could quickly
spread over the native range due to the proximity of the host trees (Anagnostakis 1987).
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2. Chestnut blight
In 1904, Herman W. Merkle at the Bronx Zoological Gardens in New York, first
documented an infection on many chestnut trees (Anagnostikas 1987). This infection,
now referred to as chestnut blight, is typically identified by its bright orange coloration
and is caused by the fungus Cryphonectria parasitica that can be spread in two forms.
The first is a dry, disc-like form (ascospores) and can be spread by the wind large
distances. The second is a smaller, sticky form (pycnidia) that is spread by fruiting
bodies after rain fall and has a relatively shorter range of dispersal. This fungus causes
large swollen or sunken orange colored cankers on the trunks or limbs of the chestnut tree
(Anagnostikas 1987). After infection, the leaves above the point of infection die,
followed by the limbs (Anagnostikas 1987, Kepenecki et al. 2004, Guidone et al. 2007).
Within a few years the entire tree is dead above the point of initial infection. The roots
and root collar appear to be resistant, and sprouting shoots from dead stumps is common,
though the blight often reappears before the new tree sprouts reach sexual maturity and
nut production (Johnson 1956, Davelos and Jarosz 2004). The blight is believed to have
been transported via nursery stock from China, and between 1904 and 1950 this disease
spread to all corners of the native range of American chestnut. The blight also attacks
several close relatives to the American chestnut including three species of chinquapin
(Allegheny chinquapin, Alder leaved chinquapin, Ozark chinquapin) (Johnson 1956,
Anagnostakis 1987, Hunt et al. 2006).
After the blight killed the vast majority of the three native Castanea trees, there
was most likely a sharp decline in chestnut weevil (Curculio spp.) numbers as well
(Johnson 1956, Menu 1993a,b). However, within a few years of bearing nuts, new
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chestnut tree orchards seem to be in danger of weevil infestation (Johnson 1956). The
only explanations posed to explain this phenomena are that chestnut weevil are capable
of surviving on chinquapins and on chestnut trees still living and producing nuts, which
includes sprouting from diseased chestnut stumps that may survive long enough to
produce nuts before dying back again (Johnson 1956). Occasionally, scattered pockets of
native chestnut trees or plantings of blight resistant varieties are located in isolated areas
in sufficient numbers to sustain active weevil populations (Brooks and Cotton 1929,
Johnson 1956).
3. Current chestnut tree research
In the Midwestern United States, the survivable range of the American chestnut
tree (C. dentata) and its close relatives Allegheny Chinquapin (C. pulmila pumila) and
Ozark Chinquapin (C. pumila ozarkensis), has been scattered if not eliminated due to the
persistent presence of a highly virulent chestnut blight, Cryphonectria parasitica (Gold
and Hunt 2002a, Hunt et al. 2004a, Anagnostakis 2005). However, current research has
established several cultivars of chestnut that are blight resistant and thus have revived the
tree crop industry of growing chestnuts in the Midwest, especially in central Missouri
(Hunt et al. 2006). Tree improvement specialists with the Center for Agroforestry at the
University of Missouri, have established an extensive chestnut production orchard to
examine nut yield, nut quality, and tree hardiness, as well as cultivar resistance to the
devastating chestnut blight that has plagued the native chestnut varieties since the early
20th century. There are three world-recognized commercial chestnut production tree
species, the Japanese, Castanea crenata, European, Castanea sativa, and Chinese
chestnut, Castanea mollissima (Hunt et al. 2006). However, current research by the
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Center for Agroforesty has identified Chinese chestnut, C. mollissima, as the best adapted
chestnut for Missouri, as it displays good hardiness and adequate tolerance to chestnut
blight. Although the Center for Agroforesty has over 50 Asian chestnut varieties under
production to establish recommendations for commercial growers (Hunt et al. 2004a,b),
current nut varieties (C. mollissima) suggested for the commercial establishment of
chestnut production in Missouri include Eaton, Peach, Gideon, Sleeping Giant, Qing and
Auburn Homestead (Hunt et al. 2006).
While relatively new to many consumers and producers in the Midwest, chestnuts
and their associated products are currently under a growing demand both domestically
and world-wide (Gold et al. 2005a,b, Hunt et al. 2006). The Center for Agroforesty at the
University of Missouri has conducted extensive and continued research concerning the
market value chain associated with the production and sale of chestnut (Castanea spp.);
moreover, the Center has focused upon three key areas, national market research,
production techniques/orchard management and increasing consumer demand and
awareness (McEowen 2005). Market analysis and economics of chestnut as a viable
horticultural crop for both commercial growers and small farmers across the United
States is ongoing, and researchers at the Center for Agroforestry have produced several
publications (see Gold and Hunt 2002a,b; Gold et al. 2005a,b, 2006). With the
resurgence of both the demand for chestnuts and the viability of chestnut production,
multiple aspects for commercial production in the Midwest need to be addressed
including control of potential pests. Insects such as the yellow neck caterpillar (Danata
ministra) and the potato leafhopper (Empoasca fabae) are pests of the foliage of chestnut,
especially younger trees, though treatment practices of regular pesticide application
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during peak outbreaks of these pests are well established and effective control measures
(Hunt et al. 2006). The single greatest insect risk to the chestnut crop comes from two
weevil species, and although there is a paucity of research in the United States on these
insects, they are often cited as devastating pests (Brooks and Cotton 1929, Johnson 1956,
Jaynes 1979, Horton and Ellis 1999, Hunt et al. 2006).
B. Biology and Taxonomy of Chestnut Weevil Pests 1. General descriptions of Curculio
Weevils from the insect genus Curculio (Coleoptera: Curculionidae:
Curculioninae) select nuts for reproduction from the following tree species: Carya
(hickory), Castanea (chestnut and chinquapin), Castanopsis (chinquapin), Corylus
(hazelnut), Lithocarpus (tanoak), and Quercus (oak). There are 27 documented species
of Curculio north of Mexico (Brooks and Cotton 1929). Johnson (1956) states that the
genus Curculio, formerly known as Balaninus, comprises a well defined group of species
which includes the nut weevils. Johnson (1956) further states that the distinguishing
phenotypic characters of the genus Curculio are the extremely long and slender rostrum,
or beak, and the vertical mandibles.
There are two primary species of nut weevils (Curculioninae: Curculio) that cause
damage to chestnut fruit during harvest in the United States, and these insects bare the
common name chestnut weevil. These weevils cause the majority of their damage during
the larval stages where they consume and nearly hollow-out the inside of the mature nut
(Brooks and Cotton 1929, Johnson 1956, Ihara et al. 2003). Males and females of the
species are readily separated by the length of their proboscis. Male chestnut weevils tend
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to have a rostrum or proboscis that is shorter than their body length, where the female’s
mouthparts and longer than her body (Johnson 1956).
The phenotypic uniformity of many of the nut weevil species has historically
made this genus difficult to study and differentiate, and as many as eight to ten synonyms
of names have been documented for the American chestnut weevil species (Johnson
1956). Curculio proboscideus was described in 1775 by Fabricius, though it is now
referred to as Curculio caryatrypes, or the greater chestnut weevil. Casey (1910)
described Balaninus auriger, but it is now referred to as Curculio sayi (Gyllenhal 1836)
or the lesser chestnut weevil. Many synonyms of both species exist and this gives
evidence for the difficulty in identification due to phenotypic variations within the
species. Consequently, as these synonyms arise in the older literature, it is often difficult
to determine what species the author was discussing unless the host plant is mentioned.
Casey (1910) and Johnson (1956) list 4 synonyms for the C. caryatrypes (the greater
chestnut weevil): C. proboscidieus, B. caryatrypes, B. hariolus, and B. cylindricollis.
Casey (1910) and Johnson (1956) list 8 synonyms for C. sayi (the lesser chestnut weevil):
C. auriger, B. auriger mollis, B. strigosus, B. algonquinus, B. acuminatus, B.
setosicornis, B. macilentus, and B. perexillis.
The greater chestnut weevil, C. caryatrypes has a slightly larger adult form than
its counterpart, the lesser chestnut weevil (C. sayi). C. sayi can also be separated from C.
caryatrypes by their smaller size, more slender form, and a tendency to be shorter in body
length with a greater curvature of the beak or rostrum (Johnson 1956). The greater
chestnut weevil is considered the largest of the curculio species in the United States and
can be distinguished from all other nut weevil species by the antennae, which have the
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second funicular segment longer than the first (Brooks and Cotton 1929, Johnson 1956).
Adult female C. caryatrypes length varies from 8 to 11 mm, with a body width of 4 to 5
mm. Adult female C. sayi length varies from 5 to 9 mm, with a body width of 2.5 to 3.5
mm. Genitalia descriptions of nut weevils are not very common, as only one of the 39
Curculio species has been studied using genitalia as a taxonomic characteristic (Johnson
1956). Brooks and Cotton (1929) state that, “the [larger chestnut weevil] is
distinguished from the lesser chestnut weevil by its larger size, more robust form,
straighter and longer beak, and, in life, by its slower movements and habits of carrying
the beak projecting more directly forward.”
During all stages or instars of development, the teeth on the mandibles of the
larvae of the greater chestnut weevil are deeper than those of the lesser (Johnson 1956).
Lesser weevil larvae have an additional and distinct adfrontal area near the fork of the
epicranial suture. The pupae of lesser chestnut weevil are missing a pair of short hairs on
the beak at the base of the antennae, which are present on the pupae of the greater
chestnut weevil (Brooks and Cotton 1929, Johnson 1956).
Johnson (1956) also describes in detail variations in sizes of larval and pupal
characteristics based on measurements to the head capsule, though again, the greater and
lesser weevil are separated by size variations.
2. Life history of the lesser chestnut weevil (Curculio sayi)
Curculio sayi (Gyllenhal) is the only documented chestnut weevil to emerge in
the spring. Upon emergence (typically during the month of May), the adults remain on
the ground for 1-2 days and then fly to the tree tops to feed on the spring catkins. After
the catkins wither, the beetles disappear and are not found on the trees again until the
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middle of August (Johnson 1956). Mating occurs on or near the chestnut trees in August
and September, with egg laying taking place soon thereafter. C. sayi is the only chestnut
curculio that is not observed to mate soon after emergence, but rather seemingly remains
celibate until fall, or roughly 3 months after its initial spring emergence. The female
typically begins to lay eggs in September, after chewing through the nut and sometimes
the bur. Larvae develop inside the nuts in about 21 days and soon after, the fully grown
larvae emerge from the chestnut and burrow into the ground to a depth of about 15 to 20
cm (Brooks and Cotton 1929, Johnson 1956). The larvae cut a circular exit hole in the
nut of about 2 mm in diameter and quickly enter the soil. Johnson (1956) reported the
emerging larvae to crawl no more than 8 cm from the point where they first contacted the
ground. They burrow straight downward with little or no lateral movement for several
centimeters and construct a smooth-walled earthen cell. The depth at which these cells
are formed varies from 5 to 25 centimeters, and one of the factors which seem to
determine the depth of the cells is the nature of the soil. Van Leeuwen (1952) reported
that 97% of the larvae are found in the first 20 centimeters of soil. Once underground,
the larvae construct earthen cells where they remain as larvae until September of the
following year, at which time about 90% pupate. Those that pupate remain in the soil for
three to four weeks prior to changing into the adult form. These adults then remain in the
soil over a second winter, and emerge the following May or June (or about 21 months
after they entered the soil). A few larvae will remain in the soil over three winters (or
about 33 months). Some of this variation in range of lifecycles may be caused by larvae
size prior to entering the soil (Menu and Desouhant 2002), as well as the depth to which
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the larvae burrow (Hall and Austin 2002). Typical weevil emergence begins early May
through mid-June (Brooks and Cotton 1929, Johnson 1956, Menu and Desouhant 2002).
3. Life history of the greater chestnut weevil (Curculio caryatrypes)
Curculio caryatrypes (Boheman) is even more poorly documented than C. sayi, as
it does not tend to exist in as economically damaging numbers as its smaller counterpart
(Johnson 1956, Hunt et al. 2006). Though very similar to C. sayi in lifecycle, C.
caryatrypes is commonly referred to as completing its life cycle in only one year, thus
passing only one winter underground in the larval stage, though the literature states the
occasional individual will periodically pass two winters underground (Brooks and Cotton
1929, Johnson 1956). The adults only emerge in the fall, between August and
September, usually just as the chestnut burs are opening. While C. caryatrypes adults
emerge in the fall, as opposed to the spring emergence of C. sayi, most other life history
characteristics are similar between these two closely related species (Johnson 1956).
Adult female C. caryatrypes deposit their eggs into the nut after chewing a small hole
through the bur, and the eggs will hatch within a few days. Larvae begin to consume the
interior of the fruit for three weeks after hatching and then emerge from the nut to burrow
into the soil. Depth of larval burrowing is estimated to be between 5 and 25 centimeters
underground (Johnson 1956).
4. Life history of the European chestnut weevil (Curculio elephas)
The chestnut weevil Curculio elephas (Gyllenhal) is an important pest of the
European chestnut, Castanea sativa. According to Menu and Debouzie (1992) and
Desouhant (1998), this species can attack chestnuts and acorns (Quercus spp.), a behavior
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not reported for the two American chestnut weevil species. Desouhant (1998) also states
that mating and egg-laying occurs soon after adult emergence, which only occurs once
and in the fall, beginning in late August and continuing through September. Larvae leave
the fruit and burrow into the ground to over-winter. This underground diapause can
extend for two, three or four winters (Menu et al. 2000, Soula and Menu 2003, Venette et
al. 2003, Soula and Menu 2005). Adult weevils live for only 28 days on average and
females can lay up to 28 eggs (Desouhant 1998, Debouzie and Menu 1992). Dispersal is
thought to be limited, as adults tend to remain in or near the chestnut trees they emerge
under (Debouzie and Pallen 1987, Venette et al. 2003). Females do not select nuts for
oviposition based on the size of the nut, nor does the presence of others eggs or larvae of
the same species deter further oviposition by another female (Desouhant 1998, Debouzie
et al. 2002).
5. Life history of the Italian chestnut weevil (Curculio propinquus)
Adult Curculio propinquus (Desbrochers), the Italian chestnut weevil, emerge in
the fall between August and mid-September. The specific emergence time may relate to
geographical position of the trees and on the amount of rainfall at the end of the summer
months (Paparatti and Speranza 2005b, Chen and Scherm 2006). Females lay their eggs
by piercing a hole in the husk of the nut with their rostrum and inserting an egg into the
hole. There are no differences between the feeding hole and the egg laying hole on the
nut. Larvae feed on the amylaceous substratum of the kernel, and generally each nut
hosts no more than 2 to 3 larvae. At the end of the larval stage, the larvae cuts through
the nut shell and drops to the ground. In Central Italy, the larva buries itself at a depth
ranging from 5 to 15 cm. The pupae appear in the soil late June and through July of the
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following summer. There is a strong synchronism between adults emerging from the soil
in the fall, and the degree of chestnut fruit ripening (Paparatti and Speranza 2005a,b).
This insect completes one life cycle per year, although some larvae remain in the ground
in the larval state for several years (prolonged diapause), as observed on C. elephas (the
European chestnut weevil).
C. Tree and Nut Damage by Curculio spp.
The primary mode of injury by the lesser and greater chestnut weevils, C. sayi and
C. caryatrypes, respectively, begins with the eggs being laid inside the kernel while the
nut is still growing, or even after the nut has fallen to the ground (Brooks and Cotton
1929, Johnson 1956). These eggs hatch within a few days (usually less than one week)
and the larvae that infest the kernel can devour the entire content of the chestnut. Any
portion of the nut which is not consumed is of little or no commercial value, thus any
level of infestation within a nut removes that nut from any possibility of sale for human
consumption (zero tolerance) (Boethel et al. 1974, Neel 1985, Collins et al. 1997).
Weevil injury varies greatly in different chestnut growing localities but the variation is
mostly due to age of the planting and thoroughness of the insect control program
(Johnson 1956, Bessin 2003). It is not unusual for 50-75% of the nuts to be wormy and
often infestation can reach 90-100% if left unchecked (Johnson 1956, Johnson 1957,
Debouzie and Pallen 1987, Horton 2005). The lesser chestnut weevil tends to do more
damage to a crop due to the fact that it can reproduce in higher numbers than the greater
chestnut weevil (Brooks and Cotton 1929, Johnson 1956, Payne et al. 1972, 1975).
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Several C. sayi larvae have been noted to emerge from a single nut, and not all
larvae make their own exit holes. Johnson (1956) noted a maximum of 26 larvae from a
single nut in Maryland. Van Leeuwen (1952) reports 58 larvae from a single Japanese
chestnut. Typically, two to four C. elephas larvae emerge per infested nut (Desouhant
1998, Menu and Debouzie 1995, Desouhant and Debouzie 2000).
Feeding damage done by adults may result in the point of entry for fungal and
yeast organisms (Johnson 1956), these organisms may cause the kernel to decay and are
seldom noticed until the nuts are placed in storage. Though feeding on the catkins in
spring is noted by several sources (Brooks 1929, Menu 1993a,b, Soula and Menu 2003),
it is doubtful that this results in any appreciable damage to the tree.
Several attempts by Johnson (1956) to produce larvae from acorns, including both
laboratory and field trials, were all unsuccessful. Therefore it seems that the lesser
chestnut weevil, Curculio sayi, cannot survive apart from their primary host, chestnut, or
their secondary host, chinquapin.
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CHAPTER II
The Seasonal Occurrence of the Adult
Lesser Chestnut Weevil, Curculio sayi, in Mid- Missouri
A. Introduction
Chestnut trees were once a dominant sight across the deciduous forest of the
eastern and central United States, but following a devastating blight in the early 1900’s,
much of the native range for this tree species has been lost (Anagnostikas 1987, Davelos
and Jarosz 2004, Anagnostikas 2005). As interest in the restoration of the American
chestnut tree increases, and as commercial production of chestnut fruit is being developed
using blight resistant cultivars from Asia, a large quantity of both native and hybridized
trees are coming into maturity and nut production (Gold and Hunt 2002a, Hunt et al.
2004a,b, 2006).
The two weevil species in the United States, the greater chestnut weevil (Curculio
caryatrypes, Boheman) and the lesser chestnut weevil (Curculio sayi, Gyllenhal) attack
ripening chestnut fruit, and they can devastate a commercial chestnut operation. Of these
two species, the lesser chestnut weevil (C. sayi) has long been reported as the most
common and most damaging chestnut pest insect species (Johnson 1956, Bessin 2003,
Hunt et al. 2006). There is a paucity of recent literature that examines the ecology and
life history of this pest insect, information that is imperative to establish basic biological
parameters in order to generate a long-term management practices. The objective of this
study was to ascertain adult emergence patterns and periods of activity of C. sayi in
central Missouri.
14
B. Materials and methods 1. Field site
This study was conducted near Glasgow (Saline County), Missouri, on a private
farmstead. Several nut trees of varying ages had been planted in the area including black
walnut (Juglans nigra), pecan (Carya illinoensis), heartnut (Juglans ailantifolia) and
several chestnut varieties (Castanea spp.); however, it should be noted that the nut trees
had not been under any type of management program (pruning, fertilization, pest control
etc) for several decades.
The chestnut trees, numbering 14 in total, were spaced at fairly even intervals of
about 7 to 10 meters and their canopies were overgrown and overlapping. It is estimated
that the trees were 40 to 50 years of age. For this study, five trees were selected to be
monitored for adult C. sayi emergence and activity. The trees were of a grafted variety,
and a forestry expert had classified them as a cross between Asian and American chestnut
species (Ken Hunt, University of Missouri). The trees were 15 to 18 meters tall, and they
produced reproductive catkins in late April and May. Annual nut drop started by late-
August and continued until early-October across the five trees used in this study.
2. Cone traps
Traps used to collect ground emerging weevil were designed after the model for
pecan weevil emergence traps described by Mulder et al. (1997). These ‘cone’ traps were
constructed with two major layers of material. The first layer was composed of
galvanized screen (0.4 cm mesh) that was cut in a half circle of a 1 meter radius (Fig. 1).
This layer was reinforced by a 76 cm wooden lath, and the straight edges were
overlapped and stapled in place to form a cone. The center, which encompasses the top
15
portion of the formed cone, was cut to remove an 8-10 cm radius section that was
replaced by adding a prefabricated, commercial boll weevil trap top. The second wall
layer was cut in a similar 1 meter radius half circle from heavy duty steel fence material;
this fencing is then bent into another cone that is placed over the top of the original mesh
screen cone, thus completing the two layer design and adding increased strength and
durability. A commercially available boll weevil trap top (Great Lakes IPM, Vestaburg,
MI) (Fig. 2) was placed on top of the cone’s opening. This top portion contains all
insects that emerged from the ground covered by the 2 meter diameter of the cage.
The emergence cone trap covered a surface area of ground equal to roughly 4
square meters, and the sturdy outer fence layer was suited to hold up against falling
branches as well as high winds (Fig. 3). All cages were fastened to the ground using bent
wire and remained in place throughout the season. Each year, the cone emergence traps
were removed for the winter and were placed the following year at a slightly different
location under the canopy of the same five chestnut trees, since no chestnuts would have
fallen under the previous years trap location and no larvae would have entered the soil in
those previously covered locations.
As Curculio sayi (and any other insects) emerge from the soil, they crawl up the
inside of the cage towards the narrow opening at the apex and into the container. The
trap is designed so that insects cannot find their way back down once inside the top
container section.
Each chestnut tree had a unique canopy cover that was assessed visually and
graphed to account for ground cover and potential nut drop. Using these canopy
estimates, emergence cone traps were evenly dispersed under the potential canopy zone
16
for nut drop from the pervious year (Fig. 4). Each tree was assigned either five or six
cone traps depending on canopy size, thus a total of 26 cone traps were used each year at
the Glasgow site. Collection cages were setup and monitored during May through
October for three years (2005, 2006 and 2007).
3. Tree-mounted circle traps
In order to monitor for weevil presence in the trees, a second trap type known as
the circle trap was employed to capture Curculio sayi as they climbed up the tree trunks.
This trap type was orginially designed for both plum curculio (Conotrachelus nenuphar)
and pecan weevil (Curculio caryae) (Mulder et al. 1997). It is composed of a large cone
of galvanized 0.4 cm mesh screen. The circle traps are stapled to the bark of the chestnut
tree trunk so as to catch insects moving up the tree trunk or branches (Fig. 5). Again, a
boll weevil trap container top is used to hold the insects that are funneled to the apex of
the trap. Because of some of the variation in lower trunk branching among the five
chestnut trees used in this study, two to four circle traps were attached per tree. Circle
traps can be useful in areas where extensive mowing or livestock make ground-based
emergence cone traps less desirable. The traps were obtained from a commercial source
(Great lakes IPM, Vestaburg, MI), and were placed at the field site in 2006 and 2007.
4. Pyramid traps
A third trap type, known as the pyramid trap or silhouette trap, was also used in
2006 and 2007. Pyramid traps, which were first developed in 1994 for pecan weevil and
plum curculio monitoring, simulate the dark upright object a weevil would interpret as a
tree trunk (Mulder et al. 1997) (Fig. 6). At the field site, the pyramid traps were placed at
17
least 10 feet from the actual chestnut tree, and spaced from each other to maximize
ground coverage in conjunction with the emergence cone traps. One pyramid trap was
associated with each of the five sampling trees (Fig. 4).
5. Data analysis
The data were analyzed using analysis of variance (ANOVA), Chi-square and T-
tests procedures (SigmaStat 2004).
C. Results 1. Adult emergence data
The 2005 data revealed a spring ground emergence of adult C. sayi, beginning the
week of May 17th (2 adults collected), peaking the week of May 23rd (19 adults), and
falling back off to low levels of emergence for most of June (averaging 1-2 adults per
week) (Fig. 7). The trap data also revealed a second ground emergence period occurring
in the fall that began the weeks of August 25th (with 7 adults) and August 30th (with 5
adults) with emergence continuing into October (averaging 1-2 adults per week). The
total number of weevils collected in 2005 was 45 adults.
In 2006, the first captured weevils in the ground emergence cone traps occurred
during the week of May 12th (18 adults) (Fig. 8). The following two weeks had 129 and
49 adults emerging from the ground, respectively. The spring emergence period
continued into June though with decreasing numbers of adult C. sayi emerging. A second
emergence period started to occur during the week of August 31st (46 adults) and
continued into October (with average emergences of 4-5 adults per week). A total of 286
adult weevil were collected in the cone emergence traps in 2006.
18
In 2007, the adult emergence began the week of May 6th (12 adults) and peaked
the following week of May 13th (27 adults) with numbers of emerging adults trailing off
into June (Fig. 9). The second emergence period of 2007 began the week of August 26th
(4 adults), peaked the week of September 2nd (12 adults), and continued to decline into
October. A total of 75 adults were collected in the ground emergence cone traps in 2007.
2. Adult activity data
Circle and pyramid traps (hereafter referred to as ‘activity’ traps) were open to the
environment and captured beetles actively moving about the field site. In 2006, these two
trap types collected 389 adults. Spring adult activity was relatively low, as only 36 adults
were collected. Peak collection weeks in the two activity traps always followed weeks of
peak ground emergence data (Fig. 8). While ground emergence numbers in the spring of
2006 peaked during the week of May 19th, adult activity numbers peaked the following
week of May 26th. More adults were collectd in the emergence traps than in the activity
traps during the spring (Fig. 8).
Fall adult activity (as measured by the two activity trap types) was much higher
than that recorded in the spring as 353 adults were collected, beginning in August and
extending through October (Fig. 8). Activity traps began capturing adult C. sayi during
the week of August 31st (12 adults) and peaked the week of September 21st (177 adults).
Adult weevils continued to be captured in relatively high numbers through October. Fall
activity trap data in 2006 were much greater than the fall emergence data (353 adults
versus 83 adults, respectively) over the three month period from August through October
(Fig. 8). Thus it appears that most of the adults at the field site had migrated into the
area.
19
In 2007, collection in the spring activity traps peaked (with 7 adults the week of
May 20th) following the spring ground emergence trap peak (of 27 adults the week of
May 13th) (Fig. 9). Spring ground emergence traps produced more adults than the
activity traps. Fall adult activity was higher than in the spring in terms of total adults
collected; however, the activity levels were not a parabolic curve as was recorded in
2006. Activity traps recorded a peak activity in the chestnut trees the week of October
17th (17 adults), though adults were collected in the activity traps in moderate numbers
from August through October (Fig. 9). More adults were captured in the activity traps in
the fall than were recorded that fall in the ground emergence traps.
D. Discussion
The spring emergence ground trap data at the Glasgow site were very consistent
over the three years of sampling, though amplitude of emergence numbers varied by year.
During 2005, the spring emergence began the week of May 17th; in 2006 it began the
week of May 12th, and during 2007, the week of May 6th. Also of similar timing was the
peak of spring emergence each year, which occurred on May 23rd (2005), May 19th
(2006) and May 13th (2007). This seems rather reasonable considering in the literature
the reported obligate nature of C. sayi diapause, a hibernation that is controlled by a
biological clock and not related to environmental cues (Menu et al. 2000, Menu and
Desouhant 2002)). It may also be the case that because of the proposed two to three year
diapause, overlapping generations of emergence may create an alternating pattern of large
(2006) and smaller (2005, 2007) emergences every other year.
Perhaps the single most interesting result of the three year study was the second
ground emergence phenomenon that occurred in the fall, an event that has not been
20
previously documented for C. sayi. This second emergence was typically observed to
begin in late August, and often continued through September and sometimes into
October. It is commonly reported in the literature that emerging adults in the spring feed
on the catkins in May and June, and then disappear for the summer months only to return
in the fall when the fruit of the chestnut tree are exposed for oviposition.
There are numerous conjectures as to what happens to the adults of the lesser
chestnut weevil that emerge in the spring time. The two most commonly reported
suppositions in the literature, though neither have any experimental evidence, are that (1)
the adults that emerge in the spring leave the site of their emergence and disperse to
neighboring areas, only to fly back in the fall to the chestnut trees (Brooks and Cotton
1929, Menu 1993a,b), or that (2) the adults bury themselves in the soil again, lie dormant
for the summer months and reemerge in the fall when the burs are opening and the nuts
are ready for feeding and egg laying (Brooks and Cotton 1929, Johnson 1956, Menu
1993a,b, Desouhant 1998).
Based on the data collected from the activity traps in 2006 and 2007, we can
elaborate further on the two aforementioned possibilities for C. sayi disappearance during
the summer months. First, note the relatively small amount of C. sayi captured by the
activity traps (the tree mounted circle traps and ground-based pyramid traps) during the
spring compared to the large ground emergence numbers recorded both in 2006 and
2007. This suggests that relatively few adults that emerge in the spring are actively
seeking the nearby chestnut trees. However, note the relatively high amount of C. sayi
captured by the activity trap types during the fall months, which is especially interesting
given the smaller amplitude of fall emergence from the soil. This strongly suggests that
21
insect activity is high in and around the chestnut trees during the fall when compared to
the activity in the spring, and moreover, that fall adult emergence alone cannot account
for the large numbers of C. sayi captured during the fall. It seems apparent that C. sayi
does in fact return in high numbers to the chestnut trees in the fall, though this evidence
does not directly address the two possibilities of summer disappearance, namely whether
or not the insects disperse by flight from the site, or rebury themselves over the summer.
The circle and pyramid trap data does indicate that adult C. sayi activity at the site during
the summer months, especially around the trunk of the trees, was relatively non-existent.
One can assume that the spring emerged weevils had left the chestnut tree area.
However, we cannot conclusively discount the possibility that the weevils spent the
summer in the trees’ canopy, since canopy samples were not taken. But the nature of the
circle and pyramid traps do strongly suggest that the adult C. sayi were moving back into
the area during the fall, being attracted to the silhouette of the trunks and being captured
moving back up the chestnut trees.
The seasonal occurrences of the European chestnut weevil, C. elephas, and the
Italian chestnut weevil, C. propinquus, are well documented and report only a single
emergence period that occurs in the fall (Menu 1993a,b, Desouhant 1998, Desouhant and
Debouzie 2000, Paparatti and Speranza 2005a,b). The seasonal occurrence of C. sayi,
which was the only chestnut weevil to have been reported in the literature to emerge in
the spring, appears to also have the unique characteristic of two annual emergence
periods. Further study is required to ascertain the ecological importance of this spring
emergence, as adults are active long before their host plant is generating nuts for female
C. sayi oviposition.
22
Figure 1. Schematic pattern and dimensions of the ground emergence cone traps (modified from traps described by Mulder et al. 1997). The flat edges are stapled together over a wooden lath to form a cone. The center of the semicircle, which becomes the top of the cone upon folding, is removed to allow the addition of a pre-fabricated boll weevil trap top to collect insects.
23
Figure 2. Commercial boll weevil trap top (disassembled) placed on top of the cone trap (see Fig. 1) (trap tops were obtained from Great Lakes IPM, Vestaburg, MI).
24
Figure 3. The emergence cone trap covered an area of ground roughly 2 square meters and stands at just over 1 meter tall. The inner galvanized mesh layer directs all emerging insects including C. sayi to the boll weevil trap top for collection. These traps were used in 2005 through 2007.
25
Figure 4. Schematic depiction of the location of the five selected chestnut trees at the Glasgow, MO, site. Tree trunks are shown as blackened circles. Trees were on average spaced 7.5 to 9 meters from one another. The emergence cones traps, shown as red crosses, were evenly dispersed under the tree’s canopy and the approximate position of the pyramid traps are shown as green triangles.
26
Figure 5. Tree mounted circle traps that were positioned to collect adult weevils as they move up the chestnut trees.
27
Figure 6. Pyramid traps act as a false tree trunk and capture the weevils as they climb upward and become trapped in the boll weevil trap at the top (traps obtained from Great Lakes IPM, Vestaburg, MI).
28
02468
101214161820
April27th
May10th
May23rd
June6th
June20th
July5th
July18th
Aug1st
Aug15th
Aug30th
Sept13th
Sept27th
Oct11th
Oct25th
Nov8th
Date - 2005
Num
ber o
f adu
lts c
aptu
red
Figure 7. Numbers of adult Curculio sayi captured emerging from the ground in 2005.
29
020406080
100120140160180200
April28th
May12th
May26th
June8th
June22nd
July 6th July20th
Aug 3rd Aug17th
Aug31st
Sept14th
Sept28th
Oct12th
Oct26th
Nov.8th
Date - 2006
Num
ber o
f adu
lts c
aptu
red
Activity TrapsEmergence Traps
Figure 8. Numbers of adult Curculio sayi captured emerging from the ground (data in blue) and crawling up the tree trunks (data in red) in 2006.
30
0
5
10
15
20
25
30
April29th
May13th
May27th
June10th
June24th
July8th
July22nd
Aug5th
Aug19th
Sept2nd
Sept16nd
Sept30th
Oct14th
Oct28th
Date - 2007
Num
ber o
f adu
lts c
aptu
red
Activity TrapEmergence Trap
Figure 9. Numbers of adult Curculio sayi captured emerging from the ground (data in blue) and crawling up the tree trunks (data in red) in 2007.
31
CHAPTER III
Within-Soil Distribution and Development of the
Lesser Chestnut Weevil, Curculio sayi
A. Introduction
Chestnut trees were once a dominant sight across the deciduous forest of the
eastern and central United States, but following a devastating blight in the early 1900’s,
much of the native range for this tree species has been lost. As interest in the restoration
of the American chestnut tree increases, and as commercial production of chestnut fruit is
being developed using blight resistant cultivars from Asia, a large quantity of both native
and hybridized trees are coming into maturity and nut production.
The two weevil species in the United States, the greater chestnut weevil (Curculio
caryatrypes, Boheman) and the lesser chestnut weevil (Curculio sayi, Gyllenhal) attack
ripening chestnut fruit and they can devastate a chestnut operation. Of these two species,
the lesser chestnut weevil (C. sayi) has long been reported as the most common and thus
most damaging chestnut pest insect. There is a paucity of recent literature that examines
the ecology and life history of this pest insect, information that is imperative to establish
basic biological parameters in order to generate a long-term management practices. The
objective of this study was to assess the soil distribution of mature larvae leaving the
chestnut and their subsequent development through time.
32
B. Materials and Methods 1. Field site
This study was conducted at the University of Missouri, Horticulture and
Agroforestry Research Center (HARC), New Franklin (Howard County), MO. Soil cages
were placed in Block E of the research center, which is a 1.2 ha block of alternate
plantings of “Red Delicious” and “Jonathan” apple cultivars. The tree planting distance
was 5.5 m rows and 7 m between rows. The trees were 6-years old, approximately 3 m
tall. With the exception of 2005, the apple trees in Block E had received irregular
applications of insecticide spray.
2. Soil cage containers
Sixteen cage containers were constructed from 25 cm diameter PVC tubes that
were cut into seven 7.5 cm tall sections. Seven ring sections, composing a total of 53 cm
in length, were fastened together using wooden support slates and held in place by wood
screws (Fig. 10). Each cage was buried so that only a small portion of the upper-most
ring was still visible (Fig. 11). The cages were buried, and filled with soil and given one
month to settle prior to introduction of the weevil larvae (Fig. 11).
3. Insects
Infested chestnuts were collected off the ground from a private farm near
Glasgow (Saline County), Missouri, during September of 2005. The chestnuts were
suspended over a laboratory table top in an open-ended box with a heavy gauge wire
mesh floor. As the larvae emerged from the nuts they fell through the mesh and into a
collection tray below. Emerged larvae were then collected and transported in plastic
33
containers stored in insulated boxes to the HARC field site. The larvae were then placed
on top of the soil in each cage and they immediately began to burrow downward. The
larvae that had emerged each day from the infested chestnuts were divided into 16
groups, and distributed evenly across each of the buried cages. This was done to avoid
any possible differences in larvae fitness based on the time of their emergence from the
nuts. By mid-November, a total of 85 larvae had been placed in each of the buried cages.
4. Sampling procedures and schedule
Forty-eight hours after the last of the larvae had been added to each cage soil
surface (November 2005) a single cage container was removed from the ground to assess
immediate larval activity. This initial tube extraction was used to ensure that larvae were
in fact tunneling and burying themselves in the ground after being released. Five cages
were removed in January of 2006, three months after they had last been stocked with
larvae, and a second set of five cages was removed in May of 2006. A single cage was
removed in April of 2007 (after seventeen months in the ground). The remaining four
cages were removed in October of 2007, almost two years after their initial burial.
Once removed, the individual cage assembly was dismantled into the seven
separate sections that correspond to the various soil depths. The soil contained in each
section was carefully broken apart by hand to reveal the presence, location and number of
C. sayi, as well as its life stage (i.e. larva, pupa or adult). Both live and dead weevils
were recorded.
34
5. Data analysis
Statistical analyses of the data included analysis of variance (ANOVA), Chi-
square and T-test procedures (SigmaStat 2004).
C. Results
In the first soil cage removed from the ground in November 2005, approximately
48 hours after the last of the 85 larvae had been placed on top of the soil, 62 larvae were
recaptured for a 72.9% survivorship. The vast majority of the larvae were located in the
first section of the tube (n = 50), within 7.5 cm from the soil surface (Fig. 12). From
Section 2 (7.5 – 15 cm depth) and Section 3 (15 – 23 cm depth), 9 and 3 larvae were
recovered, respectively (Fig. 12). The remaining four sections contained no larvae.
In January 2006, three months after initial burial, five soil cage tubes were
removed from the ground. No larvae were recovered in the deepest three sections, five
through seven (30 - 53 cm depth). From Section 1 (0 - 7.5 cm depth) an average of 21.6
larvae were recovered. From Section 2 (7.5 - 15 cm depth) an average of 33.6 larvae,
from Section 3 (15 - 23 cm depth) a mean of 19.0 larvae, and from Section 4 (23 - 30 cm
depth) an average of 1.4 larvae were recovered (Fig. 13). Mean survivorship of C. sayi
from the five soil cages in January 2006 was 88.7%. An analysis of variance procedure
(ANOVA) revealed no significant differences between sections 1-3 (where 99% of the
larvae were recovered), but these three sections were significantly different from sections
4-7 (P < 0.001). All recovered C. sayi from January 2006 were in the larval stage.
In May 2006, six months after the larvae were released into the ground, another
set of five soil cage containers were removed. No larvae were collected in sections 4-7
(23 – 53 cm depth). On average, from Section 1 (0 – 7.5 cm depth) 20.2 larvae were
35
recovered, Section 2 (7.5 – 15 cm depth) 38.2 larvae were recovered, and from Section 3
(15 – 23 cm depth) 10.6 larvae were recovered (Fig. 14). Mean larval survivorship
average for these five containers was 81.18%. All individuals recovered were still in the
larval stage.
In April 2007, seventeen months after the larvae were initially released into the
soil, a single soil cage container was removed and analyzed (Fig. 15). No C. sayi were
recovered in Sections 4-7 (23 – 53 cm depth). From Section 1 (0 - 7.5 cm depth) 12
larvae and 2 pupae were recovered. From Section 2 (7.5 - 15 cm depth) 22 larvae and 2
pupae were recovered. From Section 3 (15 - 23 cm depth) 4 larvae were recovered (Fig.
15).
During May 2007, several adults were captured by a cone trap apparatus covering
the tops of each of the buried soil cages. Adult emergence from the four remaining soil
cages began May 7th and continued through May 16th, for a total of 20 adults (Fig. 16).
In October 2007, nearly two years after the larvae were first released onto the soil
surface of the cages, the final set of four soil cages were removed from the ground. No
C. sayi were recovered in Sections 4-7 (23 – 53 cm depth). A total of 43 insects were
recovered from Sections 1-3 (0 – 23 cm depth) (Fig. 17). The vast majority of the
recovered C. sayi (n = 31) were in the adult stage, with a few in the pupa (n = 9) or larva
stages (n = 3). From Section 1 (0 – 7.5 cm depth) one pupa and three adults were
recovered (Fig. 17). From Section 2 (7.5 – 15 cm depth) three larvae, six pupae and 24
adults were recovered, and from Section 3 (15 – 23 cm depth) two pupae and four adults
were recovered. It should be noted that from these four cages 20 adults had emerged
36
earlier in the year (May). If these 20 adults are included into the survivorship of these
final four soil cage tubes, then survivorship of C. sayi after two years was 18.5%.
D. Discussion
The results of this study are consistent with that reported in the literature that C.
sayi larvae burrow to a depth of 8 to 25 cm underground (Johnson 1956). There does not
seem to be any cannibalism or direct competition for space underground, as it was
observed that several larvae seem to follow the same tunnels underground and then
branch off to form individual clumped earthen cells in close proximity to others in their
cohort. Observations made during this study and noted in the literature concerning
Curculio, illustrate that often during the exodus from the fruit or nut to the soil, several
larvae may utilize the same exit hole generated by a single larvae (Stamps and Linit
2002). Thus it again seems reasonable that larvae may share initial pilot tunnels when
burrowing underground. The lack of cannibalism or lack of competition among a cohort
has also been reported by researchers examining chestnut weevil larvae feeding within
the fruit of the chestnut tree, and that as many as eight larvae could be found inside a
given nut and the larvae would still be devoid of direct physical damage to each other
(Menu 1993a,b, Menu and Desouhant 2002).
There is a strong and consistent pattern of distribution across the first three
sections of the soil cage tubes beginning in January of 2006, and continuing through
October of 2007. The only distribution that does not match this pattern occurred in the
first sample date in November 2005. The distribution noted at this time (48 hours after
the larvae were released on top of the soil) suggests that final depth takes more than 2
days to achieve, as a skewed number of individuals were still located in the first section,
37
with very few spread out into Sections 2 and 3. All the subsequent sample evaluation
periods revealed the same even spread of insects across the first three depth sections.
No change in the life stage of the seeded larvae was observed until the April 2007
sample date (or 17 months after the larvae entered the soil cage tubes). Thus it seems that
there is at least a 17 month period that is the minimum duration C. sayi remains in
diapause underground prior to completing its lifecycle. In fact, adult emergence from the
soil cage tubes was noted a few weeks later in May 2007, when 20 adults were captured
in the emergence traps that were placed over the top of the soil cage containers.
Therefore, the first weevil to complete their lifecycle took roughly 18 months.
In the final sample of October 2007, only a handful of individual weevil were
recovered, which supports that reported in the literature where a few C. sayi emerge the
3rd year after entering the soil as larvae. The vast majority of recovered insects in
October 2007 were in the adult stage, though few recovered adults were still alive. It
should be noted that at this time many of the soil cage tubes were quite dry and the clay-
based soil was heavily solidified. There were two possible explanations for the number
of dead adults recovered in October 2007. First, had the soil been under less drought
strained conditions, the adults may have been free to emerge in the fall of 2007 through
the softened soil (Manel and Debouzie 1997, Lapointe and Shapiro 1999). The second
possibility is that these adults were going to stay underground for another winter, and
were not emerging in the fall, but rather the following spring. Regardless, it appears that
these insects were killed by soil entrapment and the ensuing desiccation.
38
Figure 10. Schematic diagram of soil cage showing the seven ring sections of PVP pipe that were attached together using two wooden support beams.
39
Figure 11. A soil cage tube assembly above ground (left) next to a buried tube (right). Each soil cage tube was filled with soil and allowed to settle for one month prior to adding the weevil larvae.
40
0
10
20
30
40
50
60
0 - 7.5 7.5 - 15 15 - 23 23 - 30 30 - 38 38 - 47 47 - 53
Soil Depth (cm)
Num
ber
of la
rvae
reco
vere
d
Figure 12. The number of Curculio sayi larvae recovered from a single soil cage container unearthed 48 h (November 2005) after the final larval introduction on the soil surface.
41
bbbb
a
a
a
0
10
20
30
40
50
60
0 - 7.5 7.5 - 15 15 - 23 23 - 30 30 - 38 38 - 47 47 - 53
Soil Depth (cm)
Ave
rage
# o
f lar
vae
reco
vere
d
Figure 13. The mean number of Curculio sayi larvae recovered from five soil cage containers unearthed approximately 3 months (January 2006) after the final larval introduction on the soil surface. Mean numbers of larvae found at each soil depth followed by the same letter are not significantly different (P < 0.05).
42
bbbb
a
a
a
0
10
20
30
40
50
60
0 - 7.5 7.5 - 15 15 - 23 23 - 30 30 - 38 38 - 47 47 - 53
Soil Depth (cm)
Ave
rage
# o
f lar
vae
reco
vere
d
Figure 14. The mean number of Curculio sayi larvae recovered from five soil cage containers unearthed approximately 6 months (May 2006) after the final larval introduction on the soil surface. Mean numbers of larvae found at each soil depth followed by the same letter are not significantly different (P < 0.05).
43
0
10
20
30
40
50
60
0 - 7.5 7.5 - 15 15 - 23 23 - 30 30 - 38 38 - 47 47 - 53
Soil Depth (cm)
Num
ber
of in
divi
dual
s re
cove
red Pupae
Larvae
Figure 15. The number of Curculio sayi larvae (blue data) and pupae (red data) recovered from one soil cage container unearthed about 17 months (April 2007) after the final larval introduction.
44
0
1
2
3
4
5
6
7
8
May 6th May 7th May 9th May 10th May 13th May 15th
Date - 2007
Num
ber
of e
mer
ging
adu
lts
Figure 16. Numbers of adult Curculio sayi emerging in May 2007 from four soil cage containers.
45
0
10
20
30
40
50
60
0 - 7.5 7.5 - 15 15 - 23 23 - 30 30 - 38 38 - 47 47 - 53
Soil Depth (cm)
Aver
age
# of
indi
vidu
als
reco
vere
d
Adult
Pupae
Larvae
Figure 17. The mean number of Curculio sayi larvae, pupae and adults recovered from four soil cage containers unearthed approximately 2 years (October 2007) after the last of the larval introduction.
46
CHAPTER IV
Assessment of the Flight Characteristics of the Lesser Chestnut Weevil,
Curculio sayi, from a Flight Mill
A. Introduction
Chestnut trees were once a dominant sight across the deciduous forest of the
eastern and central United States, but following a devastating blight in the early 1900’s
much of the native range for this tree species has been lost. As interest in the restoration
of the American chestnut tree increases, and as commercial production of chestnut fruit is
being developed using blight resistant cultivars from Asia, a large quantity of both native
and hybridized trees are coming into maturity and nut production.
The two weevil species in the United States, the greater chestnut weevil (Curculio
caryatrypes, Boheman) and the lesser chestnut weevil (Curculio sayi, Gyllenhal) attack
ripening chestnut fruit, and they can devastate a chestnut operation. Of these two species,
the lesser chestnut weevil (C. sayi) has long been reported as the most common and most
damaging chestnut pest insect species (Johnson 1956, Bessin 2003, Hunt et al. 2006).
There is a paucity of recent literature that examines the ecology and life history of this
pest insect, information that it is imperative to establish basic biological parameters in
order to generate long-term management practices. For example, with the current degree
of noncontiguous chestnut plantings across the Midwest and East coast, it is vital to
understand the flight abilities of the primary nut pest, C. sayi, and establish an
understanding of safe buffer zones between areas of infestation and clean areas of
commercial nut production.
47
Studies have been conducted on other beetle species to establish flight parameters
necessary to estimate dispersal ability, most of which concentrate on distance, duration
and speed of the insect’s flight (McKibben 1985). The use of flight mills typically
allows the tethering of an insect that can be induced to fly given the appropriate
conditions necessary for flight. Beetles that migrate or travel great distances are often
the subject of such studies, especially those that are capable of tree disease pandemics
through nematodes (Akbulut and Linit 1999) or fungal blights (Raffa and Berryman
1987). In order to assess the infestation capabilities of C. sayi in Mid-Missouri through
flight dispersal, the objective of this study was to ascertain the flight characteristics of
adult C. sayi through the use of a laboratory flight mill.
B. Materials and Methods 1. Insects
Adult Curculio sayi were collected from traps located in a chestnut tree planting
on a private farmstead near Glasgow (Saline County), Missouri. Three trap types were
used in obtaining the live weevils, including ground based cone traps, tree mounted circle
traps, and free standing pyramid traps (for descriptions of these traps see chapter II). The
adults used in this study were captured in the spring of 2007 between May and June.
Collections from the traps occurred twice per week so the adult weevils were not left in
the traps for more than 3 days at a time prior to being transported to the laboratory.
Chestnut weevils were separated by gender upon collection in the field and placed in
separate containers. Their mating status was assumed to be virgin.
Prior to their exposure to the flight mill, adults were placed in an environmental
chamber (Percival, Model I30BLL) set 24o Celsius, a photoperiod of 12:12 h (light:dark),
48
and a 30-40% relative humidity. Adult weevils remained in this chamber with access to
sugar water for 24 to 48 hours prior to being attached to the flight mill.
2. Flight mill and procedures
A flight mill that had previously been designed for the study of the flight
characteristics of a large wood boring Cerambycidae (Coleoptera) (see Akbulut and Linit
1999), was modified slightly for the smaller and lighter weight C. sayi. The radius of the
flight path, as measured along the flight mill arm, spanned 33cm; one revolution equating
to about 208 cm travelled (Fig. 18).
The mill was placed on top of a 1.2 x 1.8 meter poster board with an alternating
pattern of white and black stripes to aid in the retinal stimulation often required to induce
normal insect flight (and possibly required by the insect for continued flight). Four
incandescent lights (60 watt) were setup in a square around the flight mill, which helped
control the temperature around the flight path (Fig. 19).
While many tethers were examined (including sewing thread, fine copper wire
and insect mounting pins) it was ultimately noted that paper strips were the most effective
in inducing spontaneous flight and for allowing the insects to be removed from the tether
to rest before another flight assessment. A key component in inducing flight seemed to
be the adult weevil orientating its body angle and body direction before uncasing the
wings. Ultimately, a thin strip of paper that was cut into a point and affixed to an insect
mounting pin was the most successful in meeting these pre- and post flight needs of the
chestnut weevil. The fine, pointed tip of the paper was glued to the top of the insect head
segment, just to the rear of the antennae and compound eyes (Fig. 20). The paper was
49
then angled 45 degrees from the line of symmetry through the insect, which allowed the
paper to both be out of the way of the insect elytra once the wings were uncased, as well
as to keep the paper out of the reach of the insect legs that tend to be splayed outward
during flight (Fig. 20).
Prior to flight, tethered insects were given a small section of paper to hold onto
and maneuver. Many insects orientated themselves towards one of the four corner lights
and upon dropping the paper section began to fly. Some insects were induced to fly by
removing the paper strip, thus removing the tarsi from tactile contact. The temperature
that was maintained during each session was between 31 – 33o Celsius. Once flight
began, as defined by the uncasing of the wings, flight duration and the number
revolutions around the mill were recorded (Fig. 19).
The flight capabilities of 11 females and 10 males were assessed in this study,
with each insect being examined 2 to 3 times (repetitions). Adults were given a 5 minute
rest period between their flight attempts before the tether was removed and the insect was
returned to the growth chamber. All insects were flown between one and three days after
capture at the field site.
C. Results
The average flight duration for females was 578 seconds (9.6 minutes) (Table 1).
The average flight duration for males was 688 seconds (11.4 minutes). The male mean
flight duration was significantly greater than the female mean (P < 0.05). Maximum
flight duration for females was 7205 seconds (~ 2 hours) and for males the maximum
flight duration was 7035 seconds (~ 1.9 hours).
50
For females, the mean distance traveled was 226.6 meters, while males flew an
average distance of 247.1 meters (Table 1). There was no significant difference between
males and females for average distance traveled. The maximum distance traveled by a
female was 3044.13 meters, and for a male, the maximum distance recorded was 2517.41
meters.
Male C. sayi generated a mean flight speed of 41.39 cm/sec (n = 10), while
females had a mean speed of 41.97 cm /sec (n = 11). These two means were not
significantly different (Table 1). Female flight speed ranged from 16.34 cm/sec to 75.87
cm/sec. Male flight speed ranged from 29.14 cm/sec to 60.15 cm/sec (Table 1). It was
noted that as flights increased in duration the flight speeds decreased over time for both
males and females (Fig. 21). Female flight speed decreased at 1.13 cm/sec2 while males
flight speed decreased by 1.91 cm/sec2. Over the course of a female flight period that had
lasted 2 hr, the insect decreased its flight speed from 47.8 cm/sec to 39.6 cm/sec, or
roughly a total decrease in speed of 8.2 cm/sec. A male on the other hand, also flying for
close to 2 hours, decreased its flight speed from 50.4 cm/sec to 35.8 cm/sec, or roughly a
total decrease in speed of 14.6 cm/sec. The male was observed to decrease flight speed
over time at a higher rate than that of the female.
D. Discussion
There was not a significant difference between male and female C. sayi in mean
flight distance or speed; however, there was a significant difference between the sexes in
flight duration. The maximum flight distances recorded for both male and female insects
would allow both sexes to travel approximately between 2.5 and 3 kilometers
respectively. Interestingly, this was similar to a distance predicted by Johnson (1956):
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“That Curculio auriger (sayi) is capable of rather lengthy flight seems certain. At a Prospect
Plantation Orchard, near Graysonville, Maryland the small chestnut weevil was found for the first
time in 1949. These trees had been producing nuts for about four years. The orchard is bordered
on the west by Chesapeake Bay and the south and southeast by a rather extensive marsh. The
nearest wooded area is approximately one mile to the northeast. No chinquapin or chestnuts have
ever been known in this area. The only access to this orchard would require the adults to fly at
least two miles.”
Though average flight distances were much lower than the maximum distances observed
for both male and female C. sayi, it may be possible to repeatedly induce such lengthy
flights if preflight behavioral orientation requirements are more readily achieved in the
laboratory.
Inducing C. sayi to fly proved to be the most difficult aspect of this study. Based
on observations in the laboratory it appears that there is a cascade of necessary behaviors
required before C. sayi is willing to take to flight. Body orientations, including posturing,
angle to the light source and position of the tarsi on the preflight paper strip appeared to
be the most influential predictors of willingness to uncase the wings and begin flight
behavior. It should be noted that insects that performed flight durations of greater than
10 minutes were more apt to repeated lengthy flight durations and showed an increase in
the ease of flight inducement. Thus there may be a physiological change associated with
prolonged flight similar to that observed in migration states of some insects (Dingle
1972; Blackmer et al. 1994). Moreover, studies with female whiteflies (Bemisia tabaci)
have shown an increase in the proteins required for egg production in long-distance flyers
when compared to short-distance flyers or settled individuals. Thus, one could
52
hypothesize that a lengthy spring flight may be required for egg production in C. sayi
adult females.
Another important factor observed during this study was that any tactile
disturbance to the insect’s tarsi, mouthparts, antennae or wings resulted in immediate
halting of flight, and the beetle simply refolded its wings. Also, the insect must maintain
a certain speed around the flight mill to continue flying. If the insect cannot achieve a
moderate flight speed initially, the insect will halt flight (or the movement of the flight
mill arm). Thus there may be a correlation between willingness to fly and the speed at
which it perceives motion based on the alternating patterns of black and white stripes as it
progresses around the flight mill. This study employed a black and white pattern below
the insect to stimulate the perception of motion, though future flight studies of this insect
may benefit from a vertical wall around the flight pathway that also bares the alternating
color pattern since the field of vision of C. sayi points more towards its periphery and not
in a downward fashion. The addition of this wall, if it is able to increase the motion
perception of the insect, may both increase the ease of which the insect takes to flight as
well as increase the duration of the flight behavior once the wings are extended.
During the longer recorded flights for each gender, it was observed that flight
speed decreases linearly over time. It was also observed that the arc or distance that the
insect wing traced was reduced slowly as the insect flew for prolonged periods, which
was a good predictor of when the insect would halt flight. Moreover during prolonged
flight, while the distance the wing covered per stoke slowly decreased over time, it was
observed that occasionally C. sayi appeared to catch its second wind and regain both an
increase in flight speed as well as an increase in wing stroke distance. Flight speed in C.
53
sayi appears to be dependent on the distance covered by the wing during a single stroke,
thus again stressing the importance of tethering the insect to eliminate obstruction to the
natural flight stance and elytra positioning.
54
Figure 18. Flight mill apparatus. Enlargement window illustrates the orientation of the paper strip in relation to the tethered insect.
55
Figure 19. Top view schematic of the flight mill apparatus with the associated light/heat sources and alternating black and white stripes.
56
Figure 20. Schematic of tethering method used to attach adult weevils to the flight mill (paper strip is approximately 20 mm long).
57
Table 1. Mean flight parameters of male and female lesser chestnut weevil, Curculio sayi, based on a laboratory flight mill study. Parameters Female (n = 11) Male (n = 10) Mean Duration (in seconds) 577.6 a 687.8 b
Maximum 7,205 7,035 Minimum 13 31
Mean Distance (in meters) 226.6 a 247.1 a Maximum 3,044 2,517 Minimum 8.3 10.4
Mean Speed (in cm/sec) 41.4 a 41.9 a
Maximum 75.9 60.1 Minimum 16.3 29.1
Mean per flight parameter row followed by the same letter are not significantly different (Fischer’s PLSD, P < 0.05)
58
y = -1.1297x + 48.227
y = -1.9113x + 51.19235
37
39
41
43
45
47
49
51
300 900 1661 2280 3258 3960 5446 7035
Time (seconds)
Spee
d (c
m/s
)
femalemaleLinear (female)Linear (male)
Figure 21. Flight speed over time of male and female lesser chestnut weevils, Curculio sayi, that had flown for approximately 2 hours.
59
CHAPTER V
Thesis Summary and Conclusions
The process of pest control often begins with a foundation of ecological research
to establish baseline biological information about the target insect. Due the paucity of
recent primary literature on the chestnut weevil species found in the United States,
specifically the greater chestnut weevil (Curculio caryatrypes, Boheman) and the lesser
chestnut weevil (Curculio sayi, Gyllenhal), it was necessary to examine their ecology
thoroughly. More attention was given to C. sayi in this research as this species was
reported to be the more economically important pest. Three major objectives of this
research project were designed and conducted to address some of the basic ecological
parameters necessary to prepare long-term control measures in commercial chestnut
orchards.
First, it was established that C. sayi adults emerge twice per year, an occurrence
that was not previously noted in the literature. Spring emerging adults were captured
over the month of May, and occasionally into June. The fall emerging adults were
captured during the month of August and into early-September. Peak activity in the trees,
as measured by pyramid and circle traps, occurred in the fall. The heightened activity of
the adults present in the fall, as compared to those in the spring, coincided with the tree’s
production of nuts for oviposition. Dates of emergence and activity were consistent over
the tree years of this study (2004-2007).
Second, it was determined that C. sayi larvae burrow no more than 23 cm
underground, a finding that was supported by the current literature on the European
chestnut weevil and the older literature on the American chestnut weevil species. Based
60
on this two year study that examined the underground development of C. sayi, it seems
apparent that the minimum duration of the larval diapause is 17 months under the natural
environmental conditions of Mid-Missouri, with a few larvae requiring additional time
before they entered the pupation and adult stages.
Third, the flight capabilities of C. sayi adults, both male and female, were
estimated to be a maximum of 2 to 3 km in a single flight, though average flights were
closer to 0.25 km for both genders. The adults flew on average for 9 – 10 minutes,
though a few C. sayi flew for 2 hours. It seems apparent that this insect is perhaps better
at flight than we initially estimated, and further study is required to ascertain the extent of
its potential dispersal ability over multiple flights and a longer time frame. It should be
noted that all adults used in the flight study were collected in the spring, and that it is
possible there would be some variation in flight parameters when comparing adults
collected in the fall.
Overall, a stronger grasp of the seasonal occurrence and ecology of C. sayi in
Mid-Missouri has been achieved. Future research will progress towards establishing and
testing control measures.
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66
VITA
Ian W. Keesey was born in Madison, Wisconsin. He received his B.A. in Biology
in 2004 from Gustavus Adolphus College in St. Peter, Minnesota. He arrived in
Columbia, Missouri, during the fall of 2004 to begin his graduate studies at the
University of Missouri in the Forest Entomology Laboratory as part of the Division of
Plant Sciences. Ian has been under the direction of Dr. Bruce Barrett, his major advisor,
and presided as the president of the graduate student entomology club, the C.V. Riley
Entomological Society. His current research is focused on the ecology and behavior of
the lesser chestnut weevil, Curculio sayi, in Mid-Missouri.
Ian plans to continue on to his PhD at the University of Missouri in 2008. Future
research plans include continued work with C. sayi, with added emphasis on chemical
ecology and establishing functional control measures in chestnut tree orchards through
pheromone studies of chestnut volatiles.
67